When a power distribution circuit is designed, it is important to select a wiring size which will carry the maximum rated current for the expected service as specified by the National Electrical Code (NEC 210-9). Power is distributed by the wiring through circuit breakers which are designed to maintain contact during normal service conditions and for specified limited times under high current flow conditions, for example during start-up of an electrical motor. If the cause of an overload condition has not been corrected and the high current flow condition persists after the limited trip delay interval, the circuit breaker will automatically break the circuit.
When an electrical circuit, or appliance, shorts out, the amount of short circuit current produced can be determined by Ohms law, I=E/R, where "I" is the short circuit current in amps, "E" is the supply line voltage and "R" is the resistance of the wiring. There are three primary factors which effect the resistance R in electrical power distribution systems:
1. the type of wire (aluminum or copper); PA1 2. the size of the wire (gauge); and, PA1 3. the length of the wire. PA1 lighting fixtures; PA1 appliance internal wiring; PA1 overloaded extension cords; and, PA1 outlets which become warm. PA1 strain relief at electrical junction boxes; PA1 electrical wiring staple fasteners; PA1 appliance cords; and, PA1 extension cords.
During an electrical inspection, both the type and size of wire is checked; however, the length is difficult to check since the wiring is hidden behind walls and ceilings. Consequently, wiring length is usually not verified.
Table 1 shows the delays in tripping a circuit breaker for various lengths of wiring circuits which become shorted.
It will be seen that for wire lengths of 100 feet and over the time required to trip the circuit breaker, when a dead short is applied to the wiring or its outlet, is increased. During normal wiring installations, 250 feet of wire is consumed very quickly due to the wiring path: up a wall, across an attic, down a wall, around the room, back to the lights, or over to a switch, for example.
It should also be noted that the recent change made by the National Electrical Code in the resistance standard for wiring, permitting more impurities into the copper for ease of manufacturing, had the effect of reducing the quality of wiring systems by approximately 20%, which further reduces the short circuit current for the same length of wire, thereby increasing the short circuit response time of the circuit breaker.
After a six year study of both commercial and residential wire distribution systems, it has been determined by means of electronic measurements (See U.S. Pat. No. 4,316,187 entitled "Current Rating Verification System" issued Feb. 16, 1982) that most electrical installations have one or more circuits with wire resistance length of 250 feet and greater. Most of those installations were found to have one or more circuits with wire resistance length greater than 500 feet.
Due to the high temperature of the electrical arcs produced by electrical shorts (an electric welder, which is a form of controlled arcing, produces temperatures of 16,000 degrees F. at the point of arcing), only a fraction of a second is needed to start an electrical fire.
It is believed that electrical wiring short circuits have become the leading cause of fire in homes and office buildings due to the inability of the circuit breaker to disconnect the circuit after it shorts, before an electrical fire is started. Most short circuit overloads are due to a heat source or mechanical friction which breaks down the wire installation allowing the wiring conductors to short together. This belief has been confirmed by testing which shows that most homes and businesses (new or old) have one or more electrical circuits which, when shorted out, would require more than 10 seconds to trip the circuit breaker.
Some common places for heat related damage to wiring insulation to occur are:
Some common places for mechanical damage to the wiring insulation to occur are:
Extension cords further compound the circuit breaker problem. A 50 foot extension cord adds approximately 0.8 ohms which is equivalent to adding 250 feet of #12 wire onto the outlet wire resistance length, whereas a 100 foot extension cord adds 500 feet. With this added resistance, it is very difficult, if not impossible, for the circuit breaker to trip when an extension cord shorts out.
Conventional circuit breakers are designed to intentionally delay tripping for variable periods of time: 10 minutes at 150% overload to 0.1 seconds at 1,600% overloads (See Table 2). Accordingly, breakers will conduct overload currents for substantial intervals in excess of the normal circuit breaker rating. This delayed breaker action is sold as a feature which prevents tripping during start-up surge currents in excess of the breaker's rating. Due to this delayed tripping action, conventional circuit breakers cannot distinguish between motor start-up currents and intermittent high resistance short circuit conditions which are a primary cause of electrical fires.
Although the wiring size and circuit breaker rating may be correctly selected for a power distribution circuit, the power conductors nevertheless may be damaged by abnormal load conditions which fail to trip the circuit breaker, for example a low resistance intermittent short or a high resistance electrical short. Intermittent shorts are particularly dangerous and may occur in supply conductors of the kind used for domestic appliances which are subjected to frequent bending, as a result of which the insultation between the two supply conductors becomes damaged and allows momentary contact, so that an arcing, intermittent short circuit condition occurs.
Interconnection resistance caused by defective connections also contributes to electrical wiring fires. Faulty connections may be found at wire nuts, barrier strip junctions, receptacle connections and fuse box connections. Other sources of poor connections are bad internal contacts of a circuit breaker or switch. The problem of interconnection resistance is aggravated by the use of aluminum wiring. Aluminum wiring is subject to accelerated damage from overloads, poor connections and physical damage because of the electroylsis of junctions induced by dissimilar metal reaction. Moreover, thermal expansion and contraction cause the connections to become loose. As connections become loose, the contact resistance increases due to the reduced pressure, with intermittent arcing contact ultimately occurring as the connectors separate.
Installation of an oversized circuit breaker may also contribute to circuit overload. An oversized breaker may be installed when the correct size is not available or when the original breaker repeatedly trips. Overrating may also occur as a result of a defective breaker which will not trip. That situation is particularly dangerous since it is generally assumed (incorrectly) that an electrical circuit can be safely loaded until the circuit protector trips.
Conventional circuit breakers are not responsive to intermittent short circuit conditions and high resistance short circuit conditions as discussed above. That is, conventional, resettable circuit breakers which have a thermal-magnetic delayed trip feature are designed to accommodate mild overloads to avoid nuisance tripping, and will tolerate extreme intermittent short circuit conditions and extreme high resistance short circuit conditions which are a primary cause of electrical fires.